HEAD-FOLD TO TWELVE SOMITES 



123 



In the region of the area pellucida the entoderm is very thin; at 

 its boundary it passes rather abruptly into the large rounded vesi- 

 cular cells of the yolk-sac entoderm, which becomes continuous 

 at the margin of the vascular area with the germ-wall; the 

 latter continues to the periphery where it merges in the undifferen- 

 tiated cell-mass (zone of junction) (Figs. 68 A-68 E). The large 

 neural tube is not yet closed. Beneath the neural tube is a sec- 

 tion of the solid rod-like notochord. 



Fig. 67. — Median longitudinal section of the head of an embryo of 13 s. 



Ectam., Ectamnion. F. B., Fore-brain. H. B., Hind-brain. Inf., In- 

 fundibulum. M. B., Mid-brain, pr'c. pi., Precardial plate. T. p., Tuber- 

 culum posterius. Other abbreviations as before. 



The mesoderm (Fig. 68 A, B, C) lies between the parts already 

 named; it consists on each side of the middle line of the following 

 parts: (1) the mesoblastic somite, a block of cells that radiate 

 from a central cavity filled with irregularly disposed cells; (2) the 

 intermediate cell-mass or nephrotome, forming a narrow connect- 

 ing bridge between the somite and the lateral plate; (3) the 

 lateral plate, split into two layers, external, known as the somatic 

 layer, and internal or splanchnic layer. The cavity between the 

 two layers is the ccelome or body-cavity; it is very narrow next the 

 nephrotome, but widens as it extends laterally to the margin 

 of the vascular area, and is divided by various strands of cells 

 extending from somatic to splanchnic layers, thus indicating its 

 origin by fusion of coelomic vesicles. 



The ectoderm plus the somatic layer constitute the somato- 

 pleure, from which the body-wall, amnion, and chorion are derived, 

 and the entoderm plus the splanchnic layer form the splanchno- 



